text:
Magnetic or True heading to determine approach frequency?
Say you are approaching a class C airport where it says (0 - 180) uses frequency A, and 181-359 uses B. If your true heading is 175 but your magnetic heading is 185, which frequency would you use?

The sectors for approach frequencies are bearings, not headings.

I get that it depends on your bearing, but the definition of bearing does not include magnetic or true, so my question still remains, frequency A or B?

In chapter 5 of the aim, it reads: "The straight-in area can be further divided into pie-shaped sectors with the boundaries identified by magnetic courses TO the . . ." leaving me to believe it is in reference to your magnetic bearing...

Neither heading nor bearing, but rather radial off the airport, as the Chart Supplement will tell you:

Radar facility sectors are delineated by lines drawn on the radar scopes, and it is exceedingly unlikely for these lines to be perfectly aligned with radials off the airport. However, when you are within the "pie" formed by the specified radials, the given frequency is likely to be correct. If it is not, the controller will pass you on to the correct frequency; you should not worry too much about being overly precise.
As with everything in aviation, assume magnetic radial unless directed otherwise.

Isn't your bearing to the airport (or some specified Navaid) also determined by your physical location over the ground?  Aren't the radial from, and the bearing to, a given location, simply reciprocals?

@quietflyer I was thinking bearing from, not to.

Isn't "radial" a very VOR specific term?  (what's a radial off an airport?)  Bearing from/to is a general geographic term, and i think more correct.  (especially for a class C airport that might not be served by a VOR)

@MichaelHall and others: from the Airport/Facility Directory Legend in the chart supplement: "Frequencies will be listed, together with sectorization indicated by outbound radials, ..."


text:
Is a checkride signoff still good after a year?
I was signed off to take the private pilot check ride. I got Covid and didn’t take it
It’s been 1 yr are those sign offs still good.

I assume this is about the US? You may want to talk to your instructor, they may want to have you run through some refresher. Not passing that test also affects them...

So your saying all the sign offs are good I just need some more time is that right

This is for the FAA...
The endorsement in your log book reads:

I certify that [First name, MI, Last name] has received and logged training time within 2 calendar-months preceding the month of application in preparation for the practical test and [he or she] is prepared for the required practical test for the issuance of [applicable] certificate.

So while the endorsement does not expire per-se, you are required to have training within 2 months of the application. If you got the endorsement and applied for the practical test within 2 months, then you are OK.
However the reality is that you probably need to have a quick hour or two with your instructor again and they will re-certify the endorsement. I'm guessing your application either wasn't made, or the application itself expired.

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text:
Does it really take 9000 feet to recover from a spin in a P-51 Mustang?
I was a bit surprised to hear from this P-51 pilot (9:33 into the video) that the Mustang requires 9000 feet to recover from a spin.  This seems high to me.  The Mustang POH does not confirm this, but given the indicated recovery procedure, it seems possible:

b. RECOVERY. -- Recovery procedure is the same in both a left and right spin. Upon application of opposite rudder, the nose drops slightly and the spin speeds up rapidly for 1 1/4 turns, after which the spin stops. Rudder force is light at first, becomes very heavy for a period of about one second at the first half turn after starting recovery, then drops to zero as the spin stops. Recovery is effected in the normal manner, that is, by applying full opposite rudder followed by movement of the stick to neutral.

I've researched altitude loss in more modern GA single engine planes, and <= 1K per turn seems common.  See this discussion about altitude loss in a 77 Mooney M20J.
I've also researched altitude loss in the Mustang a bit further, but have been unable to find confirmation of this.  Now I don't imagine we have a plethora of Mustang pilots actively participating in this forum, so I'll temper my question to be, does 9000 feet seem reasonable?  If so, why?
Note: It is not my intention to call into question what the pilot in the video is saying.  He does admit he hasn't actually spun the mustang, implying he either read or heard this elsewhere.
Update, July 21st 2017
I came across this video, a VERY good 1944 briefing likely filmed for pilots who are being introduced to the P-51 (Have to admit, I never knew Lee J. Cobb was balding!) .  The pilot demonstrates both left and right spins starting at 20:58.  It is mentioned that it takes 3 turns to recover, but unfortunately, it is never mentioned how much altitude is lost.  However, I suppose it is possible that it could be estimated  by looking at the plane against the background by someone far better at this type of math than me.
I've also reached out to the Flight Chops guy to see if he or the pilot want to add their two cents to the discussion.

It may be that you'd want to practice spins at 9000 above ground level to leave enough space for recovery. Even though 1000 ft is common you generally start the maneuver at 5000ft in case it goes wrong.

9000 feet is not likely if the pilot applied recovery techniques in a timely manner.
US Army Air Force spin tests found here resulted in an average of 3000-6000 ft alt loss. There are several memos on it all resulting in similar figures.
This one dated 6/26/1943 shows 5500 feet for P-51A

This one from 4/30/1944 shows 3000-6500 ft for P-51B

The longest drop noted was on this memo from 7/10/1943 on the P-51A which indicated a single occurrence where the pilot modified his recovery technique resulting in a drop of 7500 ft.

Sometimes a minimum density is needed to increase aerodynamic damping enough so ending the spin becomes possible. However, then there is not a fixed altitude loss to stop a spin but a fixed density altitude at which spin recovery becomes possible.
I haven't spun a Mustang myself, so this is just my 2 cents to explain what might have prompted this opinion. My opinion, however, is that this particular pilot was talking rubbish.

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text:
how do people do VOR airborne check on KOCF?
Georgia has one VOR receiver check point, which is Vienna. The chart supplement says:
Vienna (Crisp County-Cordele) 116.5/VNE A/1300 Radial 226 19NM, with check point description stating over the center of NE/SW runway. I looked up in the aeronautical chart and I found the checkpoint description matches Runway 06-24 at KCKF as well as its traffic pattern alt 1300!!!
SO, here is my question. Do people really fly to KCFK and get in the patter to check VOR while airborne?

You are totally correct. Just fly the pattern and have a look at your instruments when overhead.
Have a look at 14 CFR 91.171(b)(3) for some regulatory background:

If neither a test signal nor a designated checkpoint on the surface is available, use an airborne checkpoint […] (the maximum permissible bearing error is plus or minus 6 degrees);

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text:
how can I verify the aircraft GPS is what TSO?
My understanding is that TSO-C 129() equipment and TSO-C 196() equipment are RAIM, and TSO-C145() and TSO-C146() are WAAS.
Although I can tell which RAIM/WASS an aircraft has -- by checking on the MFD, AUX, System setup page,  I have no idea which TSO-C equipment that GPS is. Are there any clue to figure it out?
Only thing I found is something mentioned in "G1000 Pilots Guide for the Cessna Nav3", and it says
"/G" may be filed for a flight plan. the G1000 system meets the requirement of TSO-C145a Class3 or TSO-C145c Class3 and ETSO-C145 class3 installations
Added comment.
Actually, a lot of people mention Aircraft flight manual or its supplement. However, when it comes to C172S-G1000, which I fly, I could not find anything mentioning TSO-C129,196,145,or 146, in a supplement book. Anyone know exact location stating GPS related to TSO-C in a handbook, aircraft manual or supplement? or whatever.

That's usuful source, but looks that 2.3 explains how to check what version of GPS the aircraft equip, not specifically mention how to check which TSO-C equipment the GPS is.

Each of the aircraft GPS require a POH/AFM supplement.  Inside this supplement, it will state what procedures are allowed to be used with this GPS.

TSO-c129 are non-WAAS receivers
TSO-c145 are WAAS compliant receivers


text:
Help identify the fan like structure on the Ju-88 A4

The Nazi Ju-88 A4 has a fan or turbine like structure at the top mid section of the plane (seen at bottom center of the image) I'd like to know its purpose and Identify it.
GremlinWranger has answered the question and would like to thank you to all

It may be helpful if you could more clearly identify exactly what in the image you're referring to as the "fan or turbine like structure". Can you edit the image to draw an arrow on it, pointing to what you're interested in?

If it is the item at this question https://aviation.stackexchange.com/a/38281/60886 this is possibly a duplicate, though the question there is JU-87

In addition of the link provided by @GremlinWranger: the wheel-shaped part behind the glass is the sense antenna of  a direction finder. The directional antenna is below the sense antenna (not visible). Well explained in this video


text:
Looking for a way to search through airfoils based on Cl/Cd at a specific angle of attack
I'm working on a university project to design a wing that has a high Cl/Cd value when placed in a wind tunnel at an angle of attack of 6 degrees and a windspeed of 25m/s.
So far I've been using airfoiltools.com and looking at the Cl/Cd values at an angle of attack of 6 degrees given a Ncrit value of 9 and a Reynolds number of 200,000. After a quick search I was able to find one that I believe is quite suitable, but I was wondering if there was a better way of searching through a database of airfoils to find specific designs that meet my requirements.

Reynolds number will be $critical$ to your design parameters.  If your chord is 0.11 m think albatross.  If it is 2 million look into full scale gliders

Just a few clarifying questions:  1) is the Reynolds # you provided based on MAC or something else?  2) will the wind tunnel model wing have boundary layer trips installed or free transition?  3) what aspect ratio is the wing?  4) why 6 deg?  Thanks.

@AeroAndy I calculated the Reynolds number from the reynolds number calculator on airfoiltools.com and likely parameters for the wing section I will be building then rounded down to the nearest hundred thousand. I'm not sure about the wind tunnel specifications I'll have to look into it. The wing section is probably going to be something like 150mm chord length and 300mm long. 6 degrees was just the arbituary value we were tasked with designing for.

https://aviation.stackexchange.com/a/105348/66495 has some pertinet information regarding Reynolds #. At Re_c of 200k, you could have a fair amount of variability in your wind tunnel results unless you strategically apply boundary layer trips or grit.

Don't know of any slick searchable database but "Theory of Wing Sections" by Abbott & Von Doenhoff might get you in the ballpark.


text:
High lift devices on stunt planes
Assume a symmetric airfoil for a stunt plane.  But, assume you'd also like to generate a higher lift coefficient than a single symmetric foil can reasonably achieve.  I picture the arrangement looking somewhat like a normal wing with a separate trailing high-lift element deployed, but with both elements un-cambered and the second element being movable to either above or below the main wing.  I think it should work better than a symmetric foil with a normal trailing edge flap.
What lift coefficient and L/D could this achieve?

Can you edit your question to make it so that a single question is clearly posed? I see "Couldn't you use a separate symmetric airfoil... ?" and "What kind of performance is possible?" I suggest you add another paragraph with a single sentence that contains the exact question you want answered. That will make it easier for folks to help out.

Try a bi-plane.  Their wing structure is also very strong.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.

Do you mean something like an aileron?

The correct terminology is an aerobatic airplane.
High-lift devices on symmetrical airfoils are not fundamentally different from those on cambered airfoils, but if you want to use them in both directions, a simple camber flap is all that remains.
Take the Mü-28 aerobatic glider as an example. It uses a symmetric Wortmann airfoil with a full-span camber flap of 25% of chord. The activation is automatic: A bob weight will move the flap to a deflection which is proportional to the load factor, so in inverted flight it moves opposite to the movement in upright flight. Also, this helps to increase the maximum load factor in high-g maneuvers.

Mü 28 in inverted flight (source). The negative flap deflection should be easy to see.

the second element being movable to either above or below the main wing

This sounds like a Junkers flap. Moving something above or below the wing will require a mechanism and will increase drag markedly. After all, Junkers only used that flap on slow designs; their faster designs all went back to conventional flaps.
In short: The added complexity will not be worth the gain. A camber flap will achieve mostly the same result for less weight and fewer failure modes.

Junkers flap on the Ju-87 (source).

Thank you. Exactly what I was looking for. In the image it looks like there is a gap between main element and flap. But, when I look up the Mü 28 the flaps look normal. I assumed you could get a higher lift coefficient with a gap, like when large planes extend landing flaps. Race cars do something similar with multi element rear wings and while they first prioritize Cl, they must worry about drag too. Maybe you could support a spaced flap with flat vertical plates that pivot about points inside the main element so the flap position and angle of attack move together.

@PeterEaymond No, the light strip that looks like a gap is only the rear part of the fixed wing which is (I guess) reflecting sunlight. It looks like the picture has been taken late in the evening with a low sun. Yes, the gap will give a higher lift coefficient but not a better lift-to-drag ratio! Race car designs follow different rules; they even accept separated flow to cut lift without moving surfaces, so total drag can be lowered when less lift is required.

@PeterKämpf, couldn't we use a combination of the two? A bob-weight together with a distance-mounted flap? I think that would take away a lot of the complexity.

@ROIMaison In principle yes, but for low actuation forces the flap must be hinged at its quarter chord. Then its nose will be above the trailing edge for cases where it better sits below it (to avoid sitting in the boundary layer of the suction side of the wing) and vice versa. I do not see an advantage in this.

I'll admit, (non-aviation-alert) what I proposed was similar to an accepted practice for solid wing sails on certain racing sailboats. A sail has to work in both directions. In lighter winds and tacking downwind they need a high CL because apparent wind speed is reduced and sail area is limited. I thought you could get high CL using low drag elements and attached flow using a detached flap. Conventional flap drag must climb pretty fast past some angle with a lower CL limit. The sharp profile transitions seem limiting.  Can someone recommend windows 11 software to play with this?

@PeterEaymond Yes, that is an interesting case. L/D is also important for sailboats - the better, the harder they can sail into the wind. Probably the gapped flap would also make sense for aerobatic planes after all, but to my knowledge nobody has tried it. Almost all planes fly exclusively right side up, and there is a mountain of literature how to design the gapped flap for one direction. BTW; gapped flaps make most sense during approach and landing when you want a lower L/D.


text:
What does "stuff it in the back right corner" mean?
I'm still catching up with Flightchops, and came across this episode about his first tail wheel solo.  As he touches down for his first landing (around 5:03 in the video) he says "Get ready to stuff it in the back right corner".  He says it again a few seconds later.
I am low time in tail draggers (about 3 hours post endorsement) and I don't remember ever hearing my instructor use this phraseology.
Question:
What does he mean by this?  Is this common aviation terminology or just something he picked up from his instructor?
I can make an educated guess that he means to deflect the stick to the rear and right to allow for a three point landing with a right x-wind, but I'd love to get confirmation of this from the experts.
Note: I did google this, and came up blank.

+1 just for googling before posting, and your educated guess.

Hm...I'll have to mention that more often in my questions.  I always research before posting...I thought I might get flicked for ONLY googling ;-)

As far as I know, it isn't a standard aviation term, it's just a casual phrase that he's using to emphasize or remind himself about the stick movement he needs. It's like your instructor telling you to "get dancing" on the rudder pedals: it's not an aviation term as such, but it's a very clear, descriptive reminder.
In a three-point landing you pull the stick all the way back after touchdown to keep the tail down. That helps to prevent unwanted pitch movements and gets weight on the tailwheel to help with directional control on the ground (as airspeed decreases and the rudder loses authority). Deflecting the stick to the right would be because of a crosswind from the right. Put those two things together, and the required stick movement is back and to the right, which is one "corner" of the range of motion it has.

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text:
Can a pilot rated in one category solo in another category without a current flight review?
Scenario:

A pilot is rated to fly gliders in the United States.
The pilot allows his glider flight review to lapse.
That pilot now decides he wants to get a type rating for the airplane category, single engine land.

Question:

Does the pilot have to be flight review current in order to solo in a powered airplane?
(Secondary question, as long as we are at it) Does the pilot have to be flight review current in order to take the FAA practical (check ride) in a powered airplane?

Note: I'm most interested in the answer under US FAA regulations, though the answer in other countries would be of interest as well.
I have spent some time looking over the FARs, specifically 14 CFR 61.56 and I cannot find anything conclusive either way.
I should mention that I suspect the answer to this question to be no, though I am not prepared to offer evidence to back this up.
I've already seen this related question asking whether a biennial flight review is valid for any airplane category, but I don't believe it's a duplicate because that only covers the situation where the pilot is rated in both catagories. This question refers to the situation where the pilot is rated in one category, but not the other.

You do require a flight review, per the Beard (2013) interpretation:

Finally, you have also asked whether a person operating in solo flight
  under a § 61.31(d) endorsement must comply with the flight review
  requirements in § 61.56(c). With a few listed exceptions, no person
  make act as pilot in command of an aircraft unless within the previous
  24 calendar months that person completed a flight review with an
  authorized  instructor. 14 C.F.R. § 61.56(c). Section 61.56(g)
  provides an exception for student pilots provided the student pilot is
  undergoing training for a certificate and has a current solo flight
  endorsement as required under § 61.87. Because this exception applies
  to student pilots, a pilot who holds a higher level pilot certificate
  and has an endorsement for solo flight under § 61.31(d) must comply
  with the flight review requirements in § 61.56 before acting as pilot
  in command of any aircraft.

Regarding the student pilot exception, others have suggested in comments that you are in fact a student pilot because you're new to the category/class. However, the interpretation says you are not:

Section 61.81 states that subpart C "prescribes the requirements for
  the issuance of student pilot certificates, the conditions under which
  those certificates are necessary, and the general operating rules and
  limitations for the holders of those certificates." As such, by its
  express language, subpart C to part 61 applies only to those persons
  who are seeking or hold a student pilot certificate.

So, since you will be acting as PIC and none of the exceptions in 61.56 apply, you need a flight review. As for a checkride, the person taking the checkride is acting as PIC (61.47(b)) so again, you would need a valid flight review.

So the pilot who last flew gliders a decade ago, has a burden to become current in gliders & take a flight review in them, before he can solo in an SEL, while his buddy going through the same program simply solos when his instructor says, go? That makes no sense.  How do you say that the prior-glider pilot is other than a student, like every other student, when it comes to what he's doing in his SEL training?

@RalphJ No one ever said the regulations have to make sense :-) Have a look at 61.81: the student pilot regulations apply only to someone who holds a student pilot certificate. Once you become a private pilot, you never hold a student pilot certificate again, therefore Subpart C becomes irrelevant. That's why 61.31 applies here, not 61.87. And as I mentioned in my answer, I couldn't find a specific article on this, so I'm fully prepared to be wrong if someone finds a better explanation.

@RalphJ It's not him saying that, it's both the FAA and the FARs. He is not like every other student because he can exercise the privileges of a licensed private pilot when he's PIC. The students cannot. So because when he solos he gets extra privileges, he also must meet additional requirements.

@RalphJ I did think of one semi-plausible reason for this. If someone's already a pilot they should have a certain level of knowledge about regulations, weather, radio communications etc. and it's unlikely that type training would cover that again, unless it's directly relevant to the new aircraft. Requiring a flight review would make sure that the pilot is at least fairly current on that knowledge: in your example, the student has just learnt all that stuff whereas the glider pilot may have forgotten it. I'm not saying I'm right, just that it seems possible.

You gotta love the FAA.  What a bunch of idiots.  I just typed up a question about this before finding it was a duplicate.  So the guy quits flying power, lets his biennial and medical lapse, and suddenly finds he's in a deep hole compared to someone else starting from scratch in gliders?  He has to either surrender his power rating, or take a biennial in some aircraft that he no longer has any wish to fly?

We deal with this in my glider club fairly often. 14 CFR Part 61.56 provides several ways to get credit for a flight review. Imagine the extreme case of a former military pilot who has only ever flown multi-engine jets - how would he/she get a flight review in any cost effective manner?
The answer to "I don't want to spend money getting current in something I'm not going to fly again" is the FAA Wings program. An actual Flight Reviews must be done in something in which you are rated. That's not true for Wings. So, with a little planning, the first few dual flights in the new aircraft can go into Wings. Combine that with some appropriate online training and boom. You have credit for a flight review.
Works great. Now, I hope that maybe someone will see this seven months after the original conversation(s).
Terry Pitts, CFIG

I'm not sure what the ediquite is for anwering your own question. However, after seeing the extent of the discussion here, I did a bit of deaper research and finally came across this from the FAA Acting Assistant Chief Counsel for Regulations:

The pilot is training to add a glider category rating to his pilot certificate. Per 61.56(c), the pilot in question must have a current flight review to perform solo flights in a glider during training. Section 61.56(g) provides an exception for student pilots, provided the student pilot is training for a certificate and has a current solo flight endorsement as required under 61.87.  A pilot who holds a higher level pilot certificate must comply with the flight review requirements in 61.56(c) before acting as pilot in command of any aircraft. The question of whether a certificated pilot needs a current flight review to perform solo flights in another category or class of aircraft is discussed in the Letter of Interpretation to Daniel Beard, January 9, 2015, a copy of which is enclosed with this response.

This, of course, deals with the reverse situation, but I would assume the same would apply for a glider ppl trying to solo for powered.
Thus I believe @pondlife is correct here and will click the check mark for that answer.

Does the pilot have to be flight review current in order to solo in a
powered airplane?

It's even worse than that.  In the US, a commercial airplane pilot with a gazillion hours who retires and lets his biennial flight review and medical certificate lapse cannot solo a glider, even under an instructor's supervision, without either taking the steps necessary to take that BFR in an aircraft for which he is already rated, or surrendering his power ratings and applying for a student pilot's license.  In other words, that pilot is in a deep hole compared to a novice who can simply apply for a student pilot's licence and then solo under his instructor's supervision.
One way out of this dilemma may be the "Wings" program, as stated in another answer.

You gotta love the FAA.


text:
Is there any disadvantage to slipping as a way to lose altitude?
I learned how to slip during my powered training as a way to lose altitude when too high and, of course, a way to track the runway center-line during a crosswind landing.
Quite a bit more emphasis was put on slipping during my glider training.  I guess the reason for this is pretty much common sense.  There is no concept of a "go-around" when landing a glider...you only get one shot.  That said, I don't practice slips often enough when soaring because purposefully losing altitude in a glider is counter-productive, except when too high in the pattern.
For this reason, I've now gotten into the habit of using a slip to descend when flying powered.  This allows me to practice slipping with the knowledge that I am guaranteed by virtue of having an engine to have the option of regaining that altitude.  Note: I usually only do this while flying solo since slipping can be a comfort issue for passengers.
This got me to wondering...
Questions:

Putting aside the comfort of passengers, Is there any disadvantage to always using a slip to descend when flying powered?
If not, why does it seem that this method of losing altitude is not used/taught more frequently?

Update after reviewing answers
I wanted to give this some time to stew before I checked an answer.  I came into this fairly (but not absolutely) certain that the short answer was no.  I think the responses confirm this.  I understand the need some have to "warn" against stalling, but I don't really see how this warning couldn't be applied to any maneuver and really has nothing to do with the fact that you are slipping,  short of making a mistake...like skidding instead.
The conclusion I'm taking from the answers is:

Slipping can be uncomfortable to passengers, pretty much for the same reason that healing in a sailboat can be perceived as uncomfortable.
Slipping is aerodynamically inefficient.  Since the objective is to lose altitude, this would appear to be a benefit, rather than a detractor.
That some planes, in some configurations, have restrictions on when slipping is allowed/recommended/optimal.
That slipping will get you down faster, thus making it preferred if getting down faster is the objective.

In reference to your second bullet point, being aerodynamically inefficient isn't really a benefit where there are better alternatives (like starting down sooner for instance).  If the assumption in this question is that you are already high on an approach and you need to lose altitude quickly (which it sounds like it is), then the question would be improved by explicitly stating that.

@Lnafziger, the question wasn't meant to be specifically about being high on approach...it was about descending in general.  I agree that slipping should not be a substitute for flying a good pattern/approach.

To address your questions: First, slips are perfectly reasonable for controlling descents. In some aircraft, for example, many Cessna 172s, are placarded warning against extended slips with full flaps.  In the 172 case, there may be some buffeting.
Years ago, slips were emphasized more than they are today. Of course there were more primary aircraft without flaps then.  So today the vast majority of primary trainers have flaps, and with time instructors have simply avoided emphasizing slips as much.  Consequently, the forward slip becomes more of a saving maneuver than a routine maneuver.
Your mileage will vary. There are some instructors who put more emphasis on slips for energy management. Most instructors will emphasize them for more "urgent" maneuvers like making a field with a simulated engine out.
Addendum #1
Being the best pilot you can means constantly developing skill and adding new tools to your bag.  Slips are normally covered when doing simulated engine outs.  I also cover them first with pre-solo students when we cover other emergencies such as flap failure (asymmetric flap extension) or electrical failures with electric flaps. Slips are also helpful when there is a cabin fire, and help clear smoke faster (it's never really fast enough) in planes with bilateral windows.
Addendum #2
Slips have an advantage over flaps, in a temporal sense. You can slip, and then take it out, then put it back in.  As much or as little as you want.  Since flap changes normally mean trim changes, and in the case of the 172 there is a flap motor limiting application and removal rates, the same cannot be said for using flaps. As an example, landing during very turbulent and gusty conditions, I might only use partial flaps, and use slips for glide path management, particularly if going to some mountain top strip with all kinds of up and down drafts influencing the path. Just another thought, for someone as they consider enhancing their proficiency with slips.

My understanding is that there was an older model 172 that prohibited forward slips with flaps, but that most of the placards are not outright prohibitions. I've never actually flown a 172 that has this placard or limitation so I've never seen it personally.

I have, and I believe the C172M (1976) was the last one so placarded. It is not a prohibition, rather an advisory to avoid, as there may be some buffeting.  I think that came from pilots being startled by a buffet, but the behavior is not extreme and I have never experienced any adverse effects. YMMV

Putting aside the comfort of passengers, Is there any disadvantage to
  always using a slip to descend when flying powered?

No not really. Practically speaking it's well within the maneuvering limits of the aircraft so you are not really going to cause any problems. The only disadvantage might be a slightly higher risk of a low altitude stall due to the uncoordinated flight and presumably low speed. Apparently some aircraft can not be slipped with flaps extended

One additional thought! Some manufacturers specifically placard their
  aircraft that slips are not allowed with flaps extended. Not sure if
  that's their lawyers or their safety folks speaking, but it is a
  reality we must acknowledge.

If not, why does it seem that this method of losing altitude is not
  used/taught more frequently?

My instructor was big on it when we practiced emergency engine outs and I received plenty of training in slipping. I'll use it on occasion if I'm really high on final but most powered planes don't have nearly the glide ratio of gliders so cutting the power is more than enough to get the plane to really come down. I would say its not used as much in powered aircraft because you have plenty of control already and you don't need to over-complicate the situation.

I wouldn't use it as a "normal" means of descent, simply because it isn't aerodynamically efficient.  You are purposefully creating more drag and lowering the glide ratio of the airplane without a good reason. You end up using more fuel when you could have just pulled the power back (using less fuel) and started a descent or glide earlier.
That being said, good job practicing it in a safe, learning environment when you don't actually need it.  This should pay off down the road when you do want to lower your glide ratio by slipping when high on final, etc.  It's a good tool to have and should be used when appropriate.
As far as those who discuss spinning from a slip, you should always maintain sufficient airspeed (just like when you aren't slipping), but please take a look at this question about what happens when you stall during a slip: What happens in a stall during a slip?.

The point of making "non shallow approaches" in a plane that doesn't drop like a brick, is to make the plane more inefficient, and have it drop like a brick. Flaps help. Spoilers help.  But both of them lower the glide ratio...does that make them unacceptable because they are also aerodynamically inefficient?

@mongo The question wasn't limited to approaches, and as I said it is a good tool and should be used when appropriate, however I stand by my statement that it should't be a "normal" means of descent (whether on approach or not).

Another disadvantage of slipping in order to lose altitude rapidly is that, at any section of the wing, the AoA is considerably increased, because the horizontal speed is reduced slightly, while the sink speed (and thus, the vertical airspeed component) becomes much higher. The combination of both factors result in an increase of the AoA, even if the attitude of the plane is kept the same, and increasing the AoA may lead to a partial or total wing stall...

This doesn't make sense. Assuming the loss of horizontal speed is negligible (say, a sustained slip), you need a constant AoA to have a constant amount of lift. The higher sink rate is due to pitching down, and have the fuselage drag due to slip prevent the increase in horizontal speed usually associated with pitching down.

When slipping, you lose altitude because the drag becomes much higher due to the increased angle of the relative wind to the body of the plane. That higher drag slows down the plane, the lift diminishes, and the plane starts to sink with no attitude change. There's no pitching down at all in sideslipping... Hence, the horizontal component of the airspeed goes down, and the vertical component goes up, increasing the AoA.

When sideslipping, it's advisable to dip the nose a bit, precisely in order to reduce the AoA and avoid a stall... But sideslipping is always done just with aileron and opposite rudder. The virtue of that maneuver is losing altitude rapidly without gaining speed...


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What is the purpose of the small vertical dorsal fin about halfway along the fuselage of the Boeing 737?
I've searched for an answer to this question on this site and others:
What is the purpose of the small vertical dorsal fin about halfway along the fuselage of the Boeing 737?
See the attached image where I have included an arrow pointing to the small fin.

It's the antenna for the Left VHF communications radio - the ones for the Centre and Right radios can be seen under the fuselage.

Many thanks for the reply. I have long wondered what they were (I once thought they might have served an aerodynamic efficiency purpose similar to the strakes seen on engine cowlings).

Comms info on the 737...  scroll down for a diagram of the antennae placements on the fuselage.
http://www.b737.org.uk/communications.htm


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Can a 121 designated SIC log Pilot In Command time under 61.51 (e)?
This may seem like an easy answer at first: no. The generally accepted rule within part 121 is that a designated SIC never logs PIC time, whether they are a FO, Captain, or Check Airman. That's the answer I'm getting from asking around my operation as well, but I'm not convinced it's accurate.
In the 1999-Carpenter legal interpretation, the FAA wrote the following (italics added for emphasis):

Dear Mr. Carpenter:

[ ... ]

Lastly, you present the following scenario: under a Part 121
operation the air carrier has designated a pilot and a copilot as
required by FAR 121.385(c). The pilot is the authorized PIC and
the copilot is the authorized SIC. The PIC is also the company
check airman. During the course of the flight, the SIC is the
sole manipulator of the controls for the flight. Additionally,
he has passed the competency checks required for Part 121
operations, at least as SIC. You ask whether the SIC can log PIC
time for that portion of the flight in which he is the sole
manipulator of the controls for the flight. The answer is yes.
There is a distinction between acting as pilot in command and
logging of pilot in command time. "Pilot in command," as defined
in FAR 1.1, "means the pilot responsible for the operation and
safety of an aircraft during flight time ....

While it is not possible for two pilots to act as PIC
simultaneously, it is possible for two pilots to log PIC flight
time simultaneously. If the pilot is designated as PIC by the
certificate holder, as required by FAR 121.385(c), that person
is PIC for the entire flight, no matter who is actually
manipulating the controls of the aircraft, because that pilot
is responsible for the safety and operation of the aircraft.
The pilot who is the sole manipulator of the controls of the
aircraft for which the pilot is rated may also log that flight
as PIC.

It seems to me that the key here is the application of 61.51 (e), a regulation governing the logging of pilot experience, to a 121 operation which requires a designation as PIC/SIC for legal responsibility for the safety of the flight. Under part 61.51 (e), the sole manipulator of flight controls may log PIC time without being the designated PIC, just like a flight instructor and private pilot seeking a higher rating. The CFI is the designated PIC in that scenario, but the PPL, who is sole manipulator of the flight controls (usually), is also logging PIC time.
To me it looks as though there's an industry-wide misunderstanding of how the FAA intended the regulation to be applied, but I think there's probably a more likely explanation - I'm missing some piece of the puzzle.
So my question is this: what is your interpretation of what I've written here? Has Carpenter-1999 been superseded by a more recent legal interpretation, was the FAA wrong in their explanation, or do we as a pilot group widely misinterpret the FARs as they apply to logging PIC time under a 121 operation?


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What is the difference between unserviceable, unusable, and not maintained?
Here are some NOTAMS for a recent flight I was planning:
LXY 03/002 LXY NAV NDB U/S 2203081935-PERM

CRS 02/008 CRS NAV NDB UNUSABLE 2202141320-2206142111EST

ANB 05/580 SJI NAV VOR NOT MNT 2205201953-2205232000EST

We all see these kinds of NOTAMS all the time. In each case, I would consider each NAVAID to be unfit for navigational planning or flight use purposes, meaning that I now consider all of them out of service and not to be used. But is there any actual semantic difference between the qualifier in these three cases? Or do they all just mean, "it's not working, don't use it!"? If so, why are three separate qualifiers used when UNUSABLE would be sufficient?

UNSERVICEABLE (abbreviated U/S) is the ICAO version of what the FAA used to call OUT OF SERVICE (abbreviated OTS). To my mind this is less clear and a step backwards, but at least it's in line with the rest of the world now.
UNUSABLE comes out to mean the same thing as UNSERVICEABLE: you cannot use the NAVAID. In my experience UNUSABLE or NOT USABLE is more common when referring to specific radials of a NAVAID where the signal is blocked by obstructions below a certain altitude, but the NAVAID itself is still operational. As you can see from your example, however, the entire NAVAID may be deemed NOT USABLE.
MNT is the abbreviation for MONITORED. To say a NAVAID is NOT MONITORED means that the fault-reporting equipment is itself UNSERVICEABLE, or that the facility responsible for monitoring that equipment is closed. Thus the NAVAID may be perfectly operational, or it may not be; the first person to know if it isn't will be the pilot who tries to use it. Procedures which depend on NAVAIDs that are NOT MONITORED may not be used for alternate-airport planning.

Can a pilot generally expect that a navigation aid which appears to be functioning will be supplying correct data, i.e. are navigation aids, whether monitored or not, designed in such a way that they will shut down in the presence of any faults that would otherwise cause them to transmit inaccurate data?

Not quite sure of that, @supercat; that would be a good question on its own. My vague understanding is that the "monitoring" equipment simply checks for the presence of any signal on the correct frequency, while actual validation of the signal must be performed from the air (i.e. Flight Check).

@supercat, ILS has a failure mode where the instrument will indicate on center, with no fault flag, but the system is actually not working. The monitoring equipment is supposed to check for this case, but without that there are known incidents where that happened.

@JanHudec: Are there fail-safes in ILS to ensure that if an antenna gets knocked out of position, it will transmit either nothing or some kind of error indication, rather than transmitting an incorrect glide slope?


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Hearing an deep hum sound, is it the reverse thrust deployment noise?
I have been hearing a deep hum sound from where I live which sounds like an Airbus 320 Family Reverse Thrust deployment noise. NOTE: However I live 15KMS+ from the Airport but the runway approach path is almost above my house.
The sound is almost the same at 5:26 in the below video. I would like to know if it is reverse thrust or something else?

Thanks!

Very few civil airliners have been certified for thrust reversal in flight, and those that were rarely used it because of the adverse effect on passenger comfort. I'd guess the sound you hear is something else, but I've no idea what.

I don’t hear any specific “deep hum” at 5:26. I think I I hear either spoilers or thrust reversers deploy at 5:36, but that wouldn’t happen in the air 15km away. Lowering flaps or gear increases drag, which makes a lot of extra noise until the plane slows down.

I grew up under the northbound final approach path for Oakland International Airport and have experienced what you describe here, where a jet shedding altitude and getting lined up for straight-in with its engines throttled back suddenly begins to produce a very loud, low-pitched "moaning" noise about 15 miles out from the threshold. As near as I could determine, that was the point in the approach where the gear went down and/or the flaps were extended all the way- so the moaning was produced by vortex shedding & turbulence from the gear and the flaps.


text:
What's the purpose of categorizing alternate airdromes?
Why do we have alternates in categories of take off alternate, enroute alternate and destination alternate. Does this really need to categorize in groups or it's just pointless issue?

No I'm just asking about categorizing. Why do we need to set alternates in theese three groups?

https://skybrary.aero/articles/alternate-aerodrome

I know that definitions completely. I'm asking about the necessity of categorizing them and making them complicated for no reason.

The elaboration below the definitions make clear why they each exist.


text:
Where can I find the text of rule that talks about responsibility of PIC on avoiding collision from fixed obstacles?
This question relates to Qatari 777 collision with pole on taxiway. I know that in this case PIC is responsible. But I can't find the text of that on annex or docs. Can you help me to find it on annex?

I doubt that there is a specific rule forbidding you from hitting obstacles, because as you said, you know the PIC is responsible. Plus it's one of those self punishing acts... Something like "operating in a careless and reckless manner" is probably the closest you will get.  Is that what you are looking for?  And what jurisdiction?

I thought that there is a rule states that the controller has no responsibility for avoiding aircraft from fixed obstacles on maneuvering area. Or sth like that.

Perhaps it was low vis and the controller cleared them onto a taxiway that wasn't appropriate for their wingspan?  As it's been pointed out, these things are investigated and there are generally multiple root causes.  If you look purely at the regs to assign blame you will miss the big picture and be ineffective at implementing corrective action.

Maybe it's better to wait for official reports. But I heard that everyday 777s taxi using this taxiway.

If you are interested in knowing the cause, then yes.  Just not sure what the existence of a “rule” might contribute to our understanding of the situation.

As I said, I was just looking for the text of the rule that I wrote it up there.


text:
What's the reason of engine roughness while leveling off from climb or recovering from stall?
I want to find out that if relative wind pressure can affect engine or not.

Float carbs can have issues from manuevering.  Some good reading here.  R/C planes also use exhaust pressure to keep positive pressure on the fuel tank and a flexible "clunk" tube to keep fuel line under the liquid level regardless of the attitude of the aircraft.

thanks alot

No, except to the extent that dynamic pressure in the cowl affects the cooling rate.  That is, the engine cools off way faster with higher indicated speed than at low indicated speed.  On a climb, if anything the cylinders will be hot, so that's not really a cylinder cooling issue.
At the end of a climb it can be from mild spark plug fouling from a long run with a fully rich mixture.
In the case of stall recovery, the engine can cool down somewhat and stumble with a sudden application of power from both plug fouling and the lower cylinder temperatures. That's why on long descents at idle you are supposed to give periodic shots of power.
Roughness can also be from carburetor ice formation, induction leaks (lean cylinder), mag problems, spark plug problems (bad resistors - I recently had to replace my Champion plugs because 6 of 8 of them had over-the-limit resistance, causing very subtle rough running problems) or bad wires.

Thanks alot, my exact problem is that, when I level off rapidly from a climb or an unusual attitude, engine starts to fail or near failure and prop speed reduction is obvious then. But I don't have this problem when leveling off from descent.

It shouldn't do that all the time. It doesn't do it on my plane. You might have a carburation issue or an induction leak or something.

@RobertDiGiovanni only if you go zero or negative G. That will make the engine flood out and it will quit until you are back to positive G.

yeah exactly when i back to +g problem is solved. but may i ask you to explain alittle abou engine flooding that you mentioned there?

When you go zero G, the float and valve in the carb "floats"' it stops regulating fuel level in the bowl. It's exactly like the float valve in a toilet tank.  The fuel sitting in the bottom of the float bowl no longer has gravity to limit what can be sucked through the metering port by the engine's vacuum, so the mixture ratio gets all screwed up and the engine dies from overrich.  You should try running leaned out while doing the maneuvers and see what happens.  Running leaned is fine as long as you don't run it at 100% power while leaned.

Thank you so much ❤


text:
A very loud, short- lived howl
Within the last eighteen months, I'd say - commercial aircraft descending into Heathrow have started to emit a terrifying, short-lived but very loud whining howl when they pass over my house. They are about 1500-2000 feet on the final approach. it's a relatively recent phenomenon, so must be caused by a modification of some sort - I've only started to hear it lately. The worst examples are really scary, it sounds like a mechanical failure of some kind. The undercarriage are coming down around this time, so it may be that - but as I say, I've only started to hear it recently - and I've lived here for over forty years... what on earth might it be? It lasts for about 5 secs max, but frequently sets my pulse racing....

Is every single aircraft making this noise? I highly doubt so, as Heathrow sees just about every active aircraft design, and there are no global modifications across them all. It would be most helpful if you could identify which models are making the noise. There are several plane tracking apps, like FlightRadar24, which will tell you the aircraft type and it's altitude, which you can match to the sound. If you give us those we might be able to provide a proper answer.

Actually, it's not EVERY  aircraft, but it's pretty common, and, as I say it's a relatively new phenomenon - I'd say post-pandemic... I've had Flight Radar for years - I haven't bothered to track the  aircraft, because, as I say - it's too frequent. Maybe I'll try...It may be a change in procedure, rather than a change in hardware... or, perhaps a particular airliner - maybe it only happens when flights cut into the holding pattern and have to descend more rapidly? I'll see if I can pin it down

This seems to be the most likely cause: https://aviation.stackexchange.com/a/96645/63967

@TimStaffell You can verify it - when you hear that howl, open Flightradar24 and check if it is one of the first three aircrafts mentioned on my list. If it is, you've got your answer.

I did as you said... first time I've heard it for a while.... it was indeed an A-220... theory holds up! Thanks!

Glad I could help, good day!

Some airplanes make strange sounds when the landing gear is deployed/stowed, or flaps are lowered/raised. The BA-146 is famous for making a loud descending boooo noise when the flaps are lowered, for example. I live under one of the approach paths for London City, and the first time I heard it I thought I was about to witness an accident, but it is perfectly normal.
So there's nothing wrong or broken, it is caused by changes in configuration.

Yeah I was seated under the wing of a 146 when the flaps came out. Scared me silly.

This plus maybe a route change that cause aircraft to perform this at a different spot (op house)

The possible sources
Multiple components on a turbine engine can produce a howl, and the three most common sources are the air inlet, the compressor, and the combustor. Please identify which one of these you are talking about.
Inlet Howl
The howl like the one on Vulcan is due to the shape of the inlet - at certain power settings, the air inrush through that uniquely shaped inlet produces the howl. But Vulcans don't operate at Heathrow, and neither do any other aircraft with such oddly shaped inlets.
Compressor Howl
This is a much higher frequency howl produced due to the leakage of air through the compressor blade tips. This is produced on all engines, but is drowned out by the fan and exhaust noise on modern turbofan engines used by the aircrafts operating from Heathrow. This howl is the predominant sound on Su-57.
Combustor Howl (Suspected)
To allow for expansion due to heating, the combustor is not directly connected to the nozzle box¹, it is connected through sealing rings² - this connection allows for the axial movement of the combustor. This along with the fact that the combustor is only supported on two points - one on the compressor side and the other on the turbine side, means that it is free to vibrate.
At certain fuel flow rates, the combustor vibration can produce a sound that can be heard from even 10-15 kilometres away, like during a GE90 startup.
On some engines, this sound can be heard at higher fuel-flow rates as well, producing a brief loud howl like on the Pratt & Whitney GTF series engines. Engines from this particular series are used on the following aircraft:

Airbus A220
Airbus A320-neo family³
Embraer E-jets
Mitsubishi MRJ's
MC-21

The first three of these do operate from Heathrow, and are likely to produce that howl as power is adjusted during the approach, and it is probably this combustor howl that you are talking about.

¹ Gases exit the combustor into this nozzle box before entering the main turbine assembly.
² The sealing rings allow for the elongation of the combustor while ensuring an air-tight seal.
³ Not all A320-neo's have this engine - there are five engines in total that power the A320, two of which power the A320-neo. The models which are powered by the P&W GTF engines are: A320-171/-271/-272/-273.


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What is the purpose of the fuel shutoff valve?
Most GA planes have a fuel shutoff valve which I don't quite understand the purpose of. Does it cut the fuel flow with the mixture.
My theory is to prevent fuel flow at earlier stage for safety reasons?

If you're changing the mixture, doesn't that indicate the fuel is reaching the carb? Typically you don't want residual fuel sitting in the carburetor as it can break down and gum it up.

Is your question inspired by the fact we don't usually have fuel cut-off valves in a car? (note to pedants: a manual one that can be operated directly by the driver).

Some years ago, my car (Trabant 601s) had a manual fuel cutoff valve. Tank above the carburetor... you could pretty much not find any gas in the tank if you forgot to close it. You could as well find few liters in the engine.

@fraxinus: I've also owned motorcycles with fuel shutoff valves.  Same reason: the tank is above the carbs, so if there's any leakage in the system, it could empty the tank.  While the updraft carb of most GA planes probably wouldn't allow fuel into the engine, driving out to the ramp and finding your plane sitting in a big puddle of avgas could ruin your day :-)

Ah, of course, I forgot the puddle of 2-stroke oil (gas evaporated, oil stayed on the ground). Yes, Trabant car had 2 stroke engine.

It's both a maintenance and safety feature.
You need a way to cut off fuel flow to the engine compartment, either to work on the engine, or because of a fire at the engine, or because you are doing a forced landing and it helps reduce the risk of your entire fuel contents seeping onto your hot engine if you bend things a bit putting it down and a fuel line gets broken.
So you will always have a shutoff valve, and it's always somewhere upstream of the firewall.

Fuel shutoff valves are a hold over from the days of float style carburetors (and necessary on any float style carb system). With a float style carb when the fuel is on the bowl will fill up until the float floats up and shuts the flow off. Since gasoline becomes a vapor at ambient temperature the small pool in the carb will be constantly evaporating. As such the float will sink and refill the bowl. Given enough time this will drain the fuel tank. This vaporous fuel also fills the intake and can cause backfires or intake fires.
The mixture is adjusted post bowl and merely leans you to the point of cutout which may not truly be a full stoppage of fuel.
For fuel injected systems or systems that use some kind of blow through carb a shutoff valve is not strictly necessary for leak reasons but JohnK makes some good points on when its used for safety reasons. Some of these systems will also allow small paths for fuel to escape so a shutoff is often implemented to ensure total fuel stoppage.

Another factor here is that a lot of planes have the fuel tanks located above the carburetor.  Obviously true of high-wing planes, but it's even true of many low-wing ones, since (unlike the downdraft carb used on most cars pre-EFI) the updraft carb of most aviation engines is located on the bottom of the engine.  So even the smallest leak in the carb float could drain a bunch of gas.  Many motorcycles have fuel shutoff valves for the same reason.

Based on experience in the automotive world, it may also act as a slight theft deterrent.

Goodness, I hope nobody who can't identify a fuel shutoff valve is going around stealing GA planes.

@Chris Especially when they are usually clearly labelled with something like FUEL SHUTOFF...

The less technical, but accurate, answer is to meet the certification requirements of the FARs.  Specifically, FAR 23.2430 is Airworthiness Standards for Fuel systems.  It reads in part:  (a)Each fuel system must-...(5) "Provide a means to safely remove or isolate the fuel stored in the system from the airplane"

Man that's an awfully broad standard.  There must be more detailed requirements somewhere else.

Not sure I agree with "there must be"

Here are two examples of high wing fuel systems. The left is a from a 1973 Cessna 177, the right from a later model Cessna 172.  You can see in the 177 the fuel cutoff is earlier in the system, before the fuel system is pressurized, while in the later plane it is after the fuel pumps.  Either way, fuel is cutoff after the header tank to limit the amount of fuel available in case of an engine fire that could result from a crash.


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Could an airliner get better fuel efficiency at higher altitude?
By higher I mean like 60k feet instead of 30k.
Seems like the rate limiting step is engine performance at 60k feet.
Are people working engines that can operate at higher altitudes? How close are we to good performance at high altitude (60k feet).
For example: Proteus high altitude aicraft

Possible duplicate: Q: Why are many jet aircraft designed to cruise around FL350-370?

The Concorde cruised at 60,000 feet, so although you do need more powerful engines they are available. Engines are not the biggest problem. The limiting factors that make flight above 45,000 feet inefficient have more to do with Mach numbers, wing dimensions and structural requirements. These are addressed by the question mins linked above.

There are several factors to take into account to explain your question, but it is a nice one.
In a cruise flight the aircraft force balance determines that the lift compensates the weight of the air plane and the engine thrust compensates the drag.
So, assuming the weight is given (which varies along the flight) the lift is determined to ensure the air plane is not falling. The lift influences the drag, the more the lift the more drag (actually a lineal variation with the variation of the square of the lift). And so, the engine compensates the drag.
So... engine thrust depends on the weight, essentially to the aerodynamic efficiency. So, my first answer, the more efficient the aerodynamic of the air plane, the higher it can fly. So, the engine is not the only factor.
However, you are highlighting an important point, the performance of gas turbine based propulsion. The problem of this propulsion is the need for an external oxidant. Essentially, the engine needs the oxygen of the atmosphere to be able to work. So that limitation requiring oxygen will make it impossible to reach much higher flight levels.
Another option is to change the propulsion concept and use an independent engine where not only fuel is carried but also oxidant is carried in the plane. The normal answer to that one is using a rocket engine. For example, the Bell-X (first manned A/C to flight faster than speed of sound) was having a ceiling of +90000 feet using rocket propulsion.

Following your explanation, the U-2 could not have flown higher than maybe 40,000 ft, which it clearly did. This really doesn't answer the question.

I am just highlighting the limitations not introducing numbers...

The Armstrong Limit for human survival, even with supplimental oxygen, is around 60k-62k feet, where the fluids that keep one's lungs working will boil away. Even with supplimental O2, a human exposed to 60k altitude pressure will live maybe a couple of minutes, and has far less time than that to get back to a liveable pressure before permanent damage occurs.
So what happens if the pressurization system fails? I see that the Proteus boasts a 'shirt sleeve' environment, but I have to wonder what the procedure is on that aircraft to deal with loss of cabin pressure, and give the crew some chance of surviving. Most likely, the crew wears pressure suits, as are required for very high altitude aircraft like the U2 and SR71.

Well, this might answer why is not recommended more than why is not possible

The question did say airliner... It is possible to fly an aircraft at over 60,000 feet. An commercially viable airliner carrying paying passengers would have a substantial liability and publicity drawback, as the consequences of cabin pressure loss would likely be the death of everyone on board.

The Concorde flew at 60,000 feet for years. This really doesn't answer the question.


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Why does ATC ask a crew who has declared an emergency if their aircraft will be overweight when landing?
I have heard ATC ask an emergency aircraft if they will be overweight when landing.
Why do they ask?
I am not asking about the consequences of an overweight landing or what procedures are required following one, but rather why ATC would ask for this in advance of the landing.
E.G.

Does ATC take it into account when offering a runway?
Does passing it on to emergency services somehow allow them to be better prepared than if they were informed after the fact?

Higher possibility of a brake fire or runway overrun is one of the reasons.

Besides, landing a highly loaded aircraft is not advisable, since –apart from the reasons stated above– a tire (or more) may easily burst if the high-speed contact is hard, and that will be a serious extra complication...

They want to know because of the consequences of an overweight landing, but you said you didn't want to know about that.

The brakes are designed for a certain amount of energy and it doesn't matter much how long time it takes for that amount of energy to be dumped into the brakes (time is too short for any significant amount of heat to transfer to air). So, unless you give it a ultra long runway, e.g. 10 minutes of deceleration time, the brake will overheat no matter what. The best ATC can do is probably to assign a clear runway so when the plane loses control after the brake overheats and blows a tire or two it won't hit anything on the ground.

@OrangeDog sorry I wasn't sufficiently clear  -  A separate question asked what are the consequences which I didn't want to duplicate.  My question was was one of timing - why ATC would want to know ahead of the landing as opposed to checking after the fact..  Answers in the other question didn't seem to indicate that knowing ahead of time would matter.

Knowing the fact after the landing seems a little superfluous; to late to use the information for improving the chances for a successfull landing.

If an aircraft encounters a serious problem quite soon after departure that forces it to land immediately, the aircraft may be above its certified maximum landing weight. This is because there is still a lot of fuel in the tanks, which adds a lot of weight.
As @RonBeyer mentioned in a comment, landing overweight can have a number of serious consequences. When landing a heavy aircraft, stopping it on the runway already requires a lot of energy (or rather, a lot of energy needs to be converted into something else). If the aircraft is heavier than it is designed to be on landing, this may cause excessive stress on the brakes. In extreme cases, this can make the brakes catch fire. Now, open flames under an aircraft that is almost fully loaded with fuel is the recipe for disaster.
In some cases, even though the crew brakes hard on landing, the aircraft may be so heavy that stopping on the runway is impossible. A runway excursion will then follow (a situation where the aircraft leaves the runway onto the grass or similar). This is not neccesarily dangerous, but it certainly can be, depending on the surrounding terrain:

(image from http://avherald.com/h?article=46f3d531. This aircraft was not overweight, but I still think the situation shown in the picture is relevant to this question.)
Obviously, in an emergency ATC, and in turn the fire and rescue crew at the airport, need to be prepared for such events. If ATC expects an aircraft will be landing overweight, getting the crew to confirm this will allow ATC to inform the fire crew that they can expect brake fires or a potential runway excursion. It can also enable the rescue leader to position the fire vehicles at the best possible position, which would be further down the runway than normal, since the landing roll will be longer. So basically, it's a way to get information to the rescue crew so that they can prepare for what might be coming.
Depending on the type of emergency, ATC will also ask the crew about other details, in some cases also with the purpose of passing this information on to the rescue crew, since the rescue crew will normally not be in direct contact with the cockpit as long as the plane is in the air. See also this related question: Why does ATC ask emergency aircraft about fuel on board?

And even if everything goes well, the brakes will be hotter than normal, which means the aircraft may need to wait a bit for them to cool down before it can taxi to the stand. Maybe even on the runway. At the very least, the ground controller will need to find a parking space for them near the runway, where they can let the brakes cool down. Worst case, they need to be towed from the runway.

So why doesn't ATC ask "are you going to crash-land, and if so how?"  or something :P  Or is the idea to offload that part of the planning from the pilots?  If so, there must be other questions about what kind of ground support might be needed.

@PeterCordes "Are you overweight" is a much simpler question, with a much simpler answer, than "Are you going to crash land and if so, how?" In an emergency, the pilot(s) doesn't want to deal with anything that's not necessary, so simple questions are preferred when information is needed to coordinate activities on the ground. ATC has a whole bunch of people who can take that simple information and formulate a rescue plan without burdening the pilot(s).

AFACT, emergency vehicles are always positioned near the approach end of the runway and chase the aircraft. This is to minimize the risk that the aircraft collides with the vehicles (in case it has lateral runway excursion).

To add to the other answer, a great example of needing to burn off fuel before attempting an emergency landing was JetBlue 292. The nose gear would not retract after takeoff and, after flying by the tower to get a visual, they determined the nose gear was stuck in a twisted way. The flight had to circle for 2 hours to burn off fuel before landing, since it was obvious the landing gear would be damaged upon doing so.

The pilots flew the aircraft, which can carry up to 46,860 pounds (21,255 kg) of aviation fuel, in a figure eight pattern between Bob Hope Airport in Burbank and LAX for more than two hours in order to burn fuel and lower the risk of fire upon landing. This also served to lighten the plane, reducing potential stress on the landing gear and dramatically lowering landing speed as well. The Airbus A320 does not have the mechanical facility to dump fuel, despite various news agencies reporting that the aircraft was doing so over the ocean.

"The Airbus A320 does not have the mechanical facility to dump fuel, despite various news agencies reporting that the aircraft was doing so over the ocean." - excellent example of how a lot of aviation related stuff is misreported in mass media.

Wow, I can't imagine having to announce to the cabin, "We're going to have an emergency landing in about 2 hours.  Just sit tight, nothing to be worried about.  (There will be plenty to worry about when we actually try to land)"

@CortAmmon It was even worse. They had satellite TV so they could watch TV news commentators talk about the plane they were stuck on. And you know how they love worst-case scenarios...

Would it have made sense for them to fly towards New York (the intended destination)?  Having the nose gear stuck down would increase drag, so they'd probably want to aim for a landing part way there, but they could at least put the fuel to good use instead of just wasting it.  Hmm, I guess if they can't make it to New York, it's probably less convenient to deal with the passengers, and they need to burden a new airport with their problem.

As a side note, most military aircraft have the ability to manually initiate a 'fuel-dump' letting them pump excess fuel out of the tanks before attempting a landing when seriously overweight.

@PeterCordes I don't see any advantage to that.  LAX is a major airport; which would make finding alternate routing for the affected passgeners much easier than whatever small location they ended up at. If something did go horribly wrong on landing, again LAX is presumably better equiped to handle it and has access to major trauma centers to treat the injured.  The same is true for access to repair facilities.  A middle of nowhere airport would be worse in almost every possible way.

@DanNeely: I was imagining they'd pick a large airport maybe half way or 3/4 of the way to NYC; one they were sure they could make it to.  Not just "wait and see where we run out of gas".  But still, the "home" airport has major advantages for passengers who want to call a friend to come pick them up or whatever.  Or if there weren't any candidate airports where that airline had repair staff / facilities.  Or is this just a case of "don't mess around when you already have one emergency", and play it as safe as possible?  Since if something else went wrong, they could end up mid-nowhere.

@DanNeely: My initial thinking was: plane still flies, you only have a big problem when trying to land.  So why just waste fuel, instead of using it to get to NYC?  But then I realized the complication of the gear being stuck down adding drag, and other factors...  Is this worth asking as a separate question?

@PeterCordes If you can't find a duplicate, it would make a good question

@PeterCordes procedures aside, put yourself in the pilots' shoes. Your nosewheel jams, and you have no clue what caused that to happen. For all you know, engines are next. You're close to a major airport, but if you land the plane now you'd be overweight and risk fire or an excursion, so you decide to fly in a pattern until you're light enough. Still, if the situation with your aircraft suddenly worsens, you'd very much want to keep open the option to land immediately, right?

@Mels: Ah yeah, that makes sense.  I was thinking that nose-wheel-stuck was known to be a problem that can just happen on some planes, and isn't usually just a symptom of a more serious problem.  But that's not a safe assumption, especially given the downside if you're wrong.  If you want to me to post a proper question so you can post the answer somewhere more people will see it, let me know. :P

Highly related Why burn fuel at departure airport instead of flying to destination before emergency landing due to deflated tire?, which was the 3rd google hit for site:aviation.stackexchange.com nose gear stuck fly airport land.  It's about the case where the gear isn't stuck down, but the accepted answer is basically the same: there might be other problems, so stay close to a good airport, like @Mels said.


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Where would you smell the extensive use of cigarettes in the cockpit of an A321?
Last night I flew on a A321 and midway during the 2.5hrs flight I went to the rear right toilet. It strongly smelt like hefty cigarette smell, but not like anybody would have smoked in there. I had the impression that the smell came out of the airvent. The cabin attendants sitting in the galley and chatting just a metre away must have noticed it, but did not react. Could used air from the cockpit get back there?

What makes you think it came from the cockpit? The rear lavatory is far away from the cockpit (about as far as you can possibly get).

Far more logical would be that somebody smoked in the rear left toilet and that the vents for the two toilets are closely connected. There are all kinds of special rules, smoke detectors, fire extinguishers, etc. (varies by plane, but I've seen mention of this stuff many times) for airplane restrooms due to the non-zero chance that nicotine-addicted smokers on a long flight will try to light up in the one place where nobody can see them.
On the other hand, while pilots do have a lot more freedom to do what they want, including smoke, than everyone else on a plane, professional requirements (you need both hands when things get busy in the cockpit) and a healthy respect for the risk of smoke (from a fire, not from a cigarette, but plenty of fires have started from a cigarette) and fire on an airplane make me think that is rather unlikely in the cockpit of an airliner.
As far as the cabin attendants not noticing it, it is quite possible that they did notice it, but unless they found the smoking gun cigarette, there isn't much they can do during a flight.


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Why don't plane seats have shoulder straps?
Why don't seats on airliners have shoulder straps, like the seats in (European) coaches often do? Wouldn't a shoulder-strap be more effective than adopting the brace position, in the event of a crash-landing?
There's no definite answer in the other question. Speculation that the brace position may be safer than a restrained torso but unrestrained head. Speculation that seats may not be strong enough to usefully hold a shoulder strap.
I'd like to know whether the lap-strap is a "legacy" pre-dating automotive shoulder-strap inertia-lock mechanisms (retained for economic reasons?), or whether lap-straps are actually known to be a safer solution in an aircraft environment (without air-bags, with a seat in front).

Perhaps it's because the main purpose of airplane seat belts is to keep passengers from bouncing around during turbulence, and a lap belt is sufficient for that?

Those European coaches only have shoulder straps on the first seat row. The rest has lap belts, just like airliners do.

I think the duplicate question covers this one. But to re-iterate, it's because the purpose of the seat belt is not to protect the human body from sudden deceleration like in a car. Therefore it is neither pre-dating modern road car seat belts for economic reason, nor because lap belts are safer. It is a different solution engineered to solve a different problem: keep passengers in their seats during turbulence. The belts on airplanes keep passengers from going up while belts in cars keep passengers from going forward.

@kevin I don't think that's right. The lap belt decelerates your lower body in a lateral deceleration, while the seat back in front of you yields and supports your upper body. Note brace position, where you pre-position your head on the seat, as well as the fact that bulkhead rows or premium seats with legroom in front have airbags or shoulder belts. In a car, there is no close, yielding, flat surface in front of you. Rather, there's a steering wheel or windshield.

@user71659 car rear-seat passengers have a seat in front that could be made flat and yielding. However, the shoulder-strap seat-belt is the usual safety measure. Same could be said of the front passenger seat were it not for the air-bag.


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How are pitch angle and pitch rate measured in an airplane?
What equipment or method is used to obtain measurements of the pitch angle (θ) and pitch rate (q) during flight?
Edit: I am asking about how the aircraft systems sense these quantities

Are you asking about how the aircraft systems sense these quantities, or are you asking how this information is presented to the flightcrew? The answer to the first question will be some arrangement or combination of mechanical or electronic gyroscopes or IRU (assuming these are measured; not all aircraft sense or display this information). Pitch rate is not usually directly presented to the flightcrew.

As the question contains "measure" and the tags "sensors" we can assume this is about the way it is measured, but a clarification would be good.

I am asking about how the aircraft systems sense these quantities

The pitch of an aircraft relative to the horizon is observed on the Attitude Indicator which also displays roll angle. These are required for IFR flight but not VFR flight here in the US. As such they are found on most but not strictly all aircraft. These are typically gyroscopically based.

(source)
Here is a basic cutaway of the interior showing where the gyro is mounted. They are often vacuum (or positive pressure) driven but can be optical or electrical as well.

(source)
Angle to the relative wing (angle of attack) is shown on an angle of attack indicator. While not required these are becoming more popular. These are controlled by a sensor on the outside of the aircraft.

(source)
The sensor that measures this on the exterior looks like this:

(source)
As far as I know there is no pitch rate info on your standard panel.

There are three principle competing electronic technologies replacing vacuum and electric powered gyros.
Higher end systems use ring laser gyros (RLG), and rely on interference pattern changes to detect motion in the axis of the ring laser. Similar, but slightly lower cost are fiber optic gyros (FOG). Lower end systems (including $20 hobby drones) are using MEMS devices.  MEMS devices can be extremely low cost and make effective accelerators.
From a current systems design standpoint, MEMS devices tend to have less bandwidth and higher noise, but for general aviation and INS/GNSS systems for general aviation, it is expected that MEMS will continue to grow market share.  They are inexpensive, environmentally tolerant, and provide adequate performance even today.
Traditionally, gyros (rather than electronic gyros and accelerometers) have been the mainstay of aircraft attitude and rate information sensing.
Expect that many systems today, and future systems will employ primarily accelerometers, and attitudinal information will be inferred.
Addendum #1
If you have a starting reference, and not allot of system noise, then a positional and acceleration reference can be established by just accelerometers. This means that the need for a "gyro" where positional or attitudinal reference isn't needed and can be derived from the accelerometer data alone. (Most likely your smartphone only has accelerometers, and derives attitude.)
Also, higher end MEMS systems may use 6 sensors, to provide finer granularity in small accelerations, compensate for some system noise, and provide higher slew rates.
Integrated MEMS IMUs with integrated GPS/GNSS receivers are available at the chip level now.  In the US, they are available in non-ITAR configurations, which eases export of integrated products.  There are also ITAR configurations which are popular in non-military applications such as survey, autonomous vehicles and aircraft.  The ITAR units require Dept of State approval for export, and are harder to integrate with non-US teammates.
Prices will continue to drop, and capabilities will continue to grow.  I believe that the steam gauge style instruments in aircraft will be virtually obsolete within the next 20 years. They will be replaced by integrated systems, and replacement instruments for backward compatibility. The cost to manufacture MEMS instruments is rapidly falling below the cost to manufacture vacuum and electric powered gyros.
ALSO, with MEMS based IMUs it is standard that the unit will provide attitude information, and rate of change information, strictly from accelerometers.  This statement may be redundant, but I wanted to make clear that as an example, pitch, roll and yaw angle (position or attitude) and rate are readily available, as well as Lat/Long type positional information.
And just one more tidbit.  When you have an integrated GPS receiver, the filtering on the GPS processing can provide very accurate velocity information (more accurate and responsive than by interpolating positional fixes). That is a nice feature about GPS and receiver signal processing.

The predominant technology today is AHRS, pronounced A-hars, for Attitude Heading Reference System.  A typical Garmin unit provides:

3-Axis Angular Rate Sensors: +/- 500 degrees per second
3-Axis Accelerometers: +/- 8G’s
3-Axis Magnetometer: +/- 2 gauss

The existing answers cover aircraft with full instrumentation, so I add my 2 cent for simpler ones.
In aircraft that have only basic instrumentation, the pilot can use the cockpit view to judge the pitch attitude. Especially in gliders, where the forward window sill is at some distance from the pilot's eyes, the height of the horizon relative to the window sill gives a very good indication of the pitch attitude.
For the pitch rate the vestibular system of the pilot is used in combination with visual cues. This works very well in VFR flight. As @Koyovis points out, very small angular accelerations are below the threshold of the vestibular system, so without visual cues spatial disorientation is very likely.

Although the vestibular system has a threshold, below which nothing is detected.

@Koyovis: Thanks, added.


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If unexpectedly in IMC, does it help to climb into clouds to get on radar?
I gather that in the flatlands, probably not. But here is a tragic accident where a VFR-only pilot attempted to navigate steep mountain passes in Utah (Evanston, Wyoming west to Coalville, Utah, and south on to Heber City).  The clouds were reported to be closing in, but he wanted to go see for himself, and  promised ATC he would quit and go back "if" (when) visibility got too bad.  He was too successful following the freeways, and got cocky. Made his turnaround decision too late, turned around successfully, and headed back out the way he came - but collided with terrain nonetheless.  Reports compared with maps make it sound like he got turned around alright using the sky over a reservoir, and my guess is he lost contact with his "concrete compass".  It was terrible judgment all around.  Shouldn't have been there.
He did have a squawk, and was periodically picked up on radar.  He was too low for radio without relays.
Would it have been a viable strategy, instead of staying low in known mountains hoping for a sliver of navigable air between clouds and rocks... to boldly climb into the clouds, get seen by ATC, get enough altitude for radio, apologize and ask to be vectored back to usable VFR territory?

Absolutely not! If a pilot is not trained for instrument flight it would be near suicide to attempt flight in cloud due to spatial disorientation. In clouds you have to trust your instruments solely and be able to interpret them, which requires special training, retraining and practice. Without these pilots do not last long in cloud, see this related question for lots more details on inadvertent flight into IMC, which is applicable to your question.
What the pilot should have done is made a precautionary landing on one of his concrete compass lines or a field while he still had the option to do so. Instead, he had get-there-itis and got himself perished as a result.

On average you have about 178 seconds to live after entering VMC if you are not IFR rated.

I've heard that statistic before @RonBeyer, but I do not know the source. It's certainly plausible. Some pilots would survive, others would end up in a spiral dive in about 10 seconds.

The source is a study done by the FAA and the AOPA.

You'd expect them to know than @RonBeyer.

I would think if you screwed up and found yourself in clouds and couldn't immediately find your way back a climb would be prudent not only for radio contact but for terrain clearance. Spatial disorientation is the big killer, but it sounds like that's not what happened to the guy in the question.  Getting lost in a mountainous area is what killed him. If he had given himself some altitude he might have been able to get back into VMC.


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Could a P-3 Orion fly off a carrier?
The C-130 has proven itself eminently capable of landing on a carrier.
When the S-2 Viking was retired, the Navy didn't really have a good replacement for anti-submarine ops - but anti-sub warfare had also fallen in importance.
Suppose there is a resurgence in need.  The Navy has an excellent ASW aircraft in its inventory; the P-3 Orion (modded L-188 Elektra).  It's not made for carrier operations, but then, neither was the C-130.
Suppose we sort out the smaller details, like making sure the P-3 can refuel from carrier-based aircraft like the F-18 (and thus can take off and immediately tank).  This must be a rolling-on-tires operation; cat launch or tailhook recovery is out of the question.
If the captain can guarantee 35 knot headwind, could the Orion do a "rolling on pavement" takeoff and landing off the carrier?  If needed, it can use the whole deck, obviously it would work better operationally if it could use the diagonal strip.

Did the C-130 carry any meaningful load when flying off the deck? Even if the P-3 was capable of doing this, the question is can it do so and carry a mission load (except fuel), right?

Good point, it may not need a full mission load if it can just radio the carrier and have another aircrafft bring a torpedo.  Still needs sonobuoys...

P3 is deprecated for the P8, and local ASW operations went to helicopters.  I am a former P3 crew member and a cat launch in a P3 is not something I signed up for.

@guycole Thank you for your service.  Edited to clarify "no cat".  Any idea what the weight of the P-3 is, full sensor/sonobuoy load, zero fuel, zero weapons?

@Harper Sorry, I don't know what the ASW aircraft weigh.  I flew w/VQ in EP3 and we were notoriously heavy.  Sorry, I cannot be more specific than "notorious".

I think the Viking was an S-3. The S-2 was the Tracker, which the Viking replaced.

Well, going beyond just judging from the Electra's itty bitty stubby wings, digging around for Electra performance data I found a page that gives a takeoff distance of 1290 meters, or 4200 feet, which would most likely be the distance at sea level, standard day, max gross.  Which is what I would expect with such itty bitty stubby wings.
Even allowing for more power in the military versions, I'd say it's safe to go with no.

1290 meters could be reduced with JATO bottles though. As was done with C-130s for example.

But you would want something that can take off in under 2000 ft in the first place, like the Herc.  However, stick on enough JATO bottles and you can launch the carrier itself in the  air!

Add foldable wings and the weight that comes with them, arresting gear, ...


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Is the ISA still up to date?
The Standard Atmosphere dates back to 1962 when Earth was colder. The 1976 update didn't make changes within 50 km altitude (the tropo- and stratosphere). It assumes 15°C at sea level, decreases until minus 56.5° at 11 km (36,000 ft) and keeps that value until 20 km (66,000 ft) from where temperature increases.
Now I wonder whether 15°C at sea level is still the appropriate temperature to be set. It's a value that would be reached at 45th latitude from late fall to early spring but is it really appropriate to use a model that cold, especially since Earth has become warmer? Take the surface pressure on Mount Everest's summit for instance: in spite of an absolute elevation of 29,032 ft, even in January the summit of Everest is barometrically lower than that, it doesn't ever reach an altitude as high as FL290, i.e. a pressure as low as 4.575 psi. Mt Everest's average surface pressure is slightly below 5 psi, making Mt Everest actually slightly higher than 27,000 ft in the International Standard Atmosphere. If your plane's altimeter shows 28,000 ft, you're able to fly over the summit of Mt Everest.
Now let's take a look at the Chimborazo which is on the very equator and so the temperature doesn't change but remains the same throughout the year. Despite having an absolute elevation of 20,548 ft the barometric altitude of Chimborazo's summit is actually around 19,000 ft, again more than a thousand feet lower than its absolute elevation. That's because Ecuador's sea level temperature is higher than 15°C.
Now you may say "But around 45th latitude the average temperature is still around 15°, and closer to the poles it's even colder". However, noone lives on or too close to the poles, and main climbing and vacation seasons are in summer when the atmosphere is more stretched and mountains have lower barometric elevations in the ISA than they actually have above sea level, and so the altitude shown in altimeters isn't the same as the absolute altitude. That's why from a practical point of view, I see more sense in setting sea level temperature to 20° or 25° as more appropriate perhaps, and setting the pressure altitude from that warmer value.
Another part of the ISA which I don't quite agree with is the alleged isothermal layer from 11 to 20 km because I find it oversimplified: it doesn't exist like this. Temperature just decreases much more slowly beyond 11 km until an altitude around 16 km (52,500 ft), from there it increases again, and above 20 km just more quickly. And considering the Earth having gotten warmer, this may change upwards as well.
Did the Earth get warm enough so that the ISA should update their pressure values for an Earth with a sea level temperature at 20-25°C ? Could it be realized easily with the altimeters of aircraft to be readapted?

Your title question kind of belongs on EarthScience.SE. A more appropriate question for Aviation.SE would be how bigger and bigger deviations from ISA would negatively affect aviation.

@Bianfable Well, other than not showing the absolute altitude (if you needed it for some reason) I don't see any negative effects on aviation. The altitude shown is actually too low. It would be a problem if it was shown too high, e.g. FL300 despite Mt Everest being 29,032 ft might result in a crash into the mountain. However, the contrary is the case. If users think it's more appropriate for Earth Science, feel free to migrate the question there.

In any case, the satellite temperature data shows the warming phase that started in the 70s stopped about 15 years ago and we may be entering a cooling phase like the one between 1940 and the mid 70s, or worse, a return to the mini ice age that bottomed out in the early 1800s.

@JohnK That's not really true for surface air temperature. The Temperature Anomaly w.r.t. 1951-80 reached its highest value (1.01°C) in 2016 and again in 2020. 15 years ago (2007) it was only 0.66°C. Source: NASA

@JohnK But so far the cooling phase didn't start (or didn't properly as yet) and in the mediterranean there are temperatures over 35°C. That means a plane's altimeter (when set to 29.92) will show a much lower altitude than the absolute altitude it is at.

@Bianfable the satellite measured upper atmospheric temperatures are the only ones with any reliability.  The next most reliable are balloon radiosonde temperature measurements that go back to the 50s.  Surface temperature measurements are hopelessly corrupted and manipulated (recording stations near heat sources, yadda yadda yadda), and if not confirmed by atmospheric measurements, are worthless.

@HiddenUser the thing is that the errors from deviation from ISA are shared by all the other local altimeters, so a few feet deviation from perfection here or there are not important.  The only time it really counts is during instrument approaches where colder temperatures causes errors that could reduce obstacle clearance, but the temperature has to be below freezing before you even bother having to compensate for that.  A couple degrees here or there makes no difference to anything.  It's like fussing over a degree or two of magnetic variation change - not that critical.

@JohnK It is especially important for mountaineering such as where altitude sickness starts. E.g. it is often said altitude sickness starts at 8,000 ft which is about 10.9 psi in the ISA. However, on a hot summer day 10.9 psi will be at a higher altitude, around 9,000 ft perhaps so that climbers at 8,000 ft don't have to worry. And if you can fly over Everest or Chimborazo at lower flight levels than their elevations are, this is weird. Perhaps one should do the other way: define the mountain's elevations at their average barometric altitudes, but that would be even weirder.

@HiddenUser - I think altitude sickness varies widely among different people.  Some climbers have climbed Everest without oxygen.  And the FAA only starts to impose oxygen requirements above 12,500 feet.

@Jim The FAA recommends using oxygen at FL100 (~10,000 ft), requires it using for the pilots at FL125 when flown longer than half an hour there and requires its use at FL140 when flown there any time. At FL150 everyone on board must mear oxygen masks if the plane isn't pressurised. I think that's the rules, but as discussed above the flight level is often lower given than the absolute altitude is. Night vision is impacted at 5,000 ft above sea level already.

@HiddenUser The FAA requirements are all in pressure altitudes. for the FAA the flight levels don’t start until 18000 feet. But that’s beside the point.  ISA is just an agreed upon standard definition.  Nothing says that the temp and pressure at sea level has to be 15 and 29.92 at sea level anywhere.  Now if the lapse rates were to become substantially different then they’d probably want to makes some updates.

@Jim Yeah but the pressure altitude above 29.92 is the same as the flight level. I don't know, perhaps 20 or 25°C would be a better sea level temperature to use.

Yes, it is up to date in the de jure sense that no updates were formally accepted. There are separate models for 'hot' and 'cold' days (and other conditions), but they have limited use.
There are several points to understand regarding the ISA model.

First and foremost, this is a deliberatly simplistic model, even for the knowledge of the day it was proposed. Its purpose is not to describe the atmosphere for scientists, but to provide a common shared standard, primarily for engineers.
Having a common standard makes things compatible and easily comparable around the world. For example, when an aircraft is tested, the conditions on the day never match ISA. But the results are then recalculated to ISA, so that the results of other aircraft, and even of the same aircraft tomorrow, were directly comparable. And given that the model is simple, it is relatively easy to do.

The surface temperature of 15°C was chosen as the closest 'round' number to the global average surface temperature on Earth. In fact, at the time the standard was developed, this temperature was closer to 14° C, and now it probably matches better.

In addition, the model is deliberately middle-latitude one. Typically, at the equator, despite higher surface temperature, temperature falls faster (and for longer), making the stratosphere typically colder. The opposite happens at the poles. The ISA model tries to average both cases out.

The model is noticeably conservative with regards to pressure (as you pointed out yourself), providing an additional safety margin for aviation to clear obstacles in most practical conditions. (I'm not sure this was a specific reasoning for choosing the parameters, though).

As always, changing an established standard, even a deficient one, is a very big deal that can cause numerous problems and even accidents. Standards are always conservative.


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Why can canard wings not be used in all sonic aircraft despite their high control?
As Canard serves two purposes; it improves aircraft control, also contributes to lift, replacing the horizontal stabilizer and - theoretically - reducing overall drag.

What do you mean by sonic?

Canard is only more efficient on aerodynamically unstable designs. We have a detailed explanation why somewhere around the site.

The forward canard does not replace the horizontal stabilizer, it actually forces a larger one for the same directional stability$^1$.  It is also less efficient than the main wing at producing lift, essentially re-creating a biplane$^2$.
Studying the progression of the Wright designs (and others) sheds light on why canards fell out of favor and more conventional tractor designs with pitch control aft became the norm after 1910.
The early canard designs were particularly dangerous when both wings stalled, leading to an unrecoverable nose up descent.  Interestingly, when early planes only could get a few feet off the ground, this type of stall was favorable because it tended to produce less damage than crashing nose first.
$^1$ canards can be used with delta wings, which are very stable in pitch
$^2$ positive lift from a canard could produce some fuel savings at cruise if the rear horizontal stabilizer/stabilator is set at 0 degrees and the wing is set at optimal AoA.  Essentially, the elevator is taken off the back, where it usually produces downforce, and is put on the front, where it can produce up force.  The benefit must be balanced with extra weight and complexity.

They can be used on any aircraft designed for canards.
Retrofitting “all” supersonic aircraft with canards would be impractical for many reasons.